Separate out and move chunk management (creation/desctruction and
[de]population) code into percpu-vm.c which is included by percpu.c
and compiled together.  The interface for chunk management is defined
as follows.

 * pcpu_populate_chunk		- populate the specified range of a chunk
 * pcpu_depopulate_chunk	- depopulate the specified range of a chunk
 * pcpu_create_chunk		- create a new chunk
 * pcpu_destroy_chunk		- destroy a chunk, always preceded by full depop
 * pcpu_addr_to_page		- translate address to physical address
 * pcpu_verify_alloc_info	- check alloc_info is acceptable during init

Other than wrapping vmalloc_to_page() inside pcpu_addr_to_page() and
dummy pcpu_verify_alloc_info() implementation, this patch only moves
code around.  This separation is to allow alternate chunk management
implementation.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Howells <dhowells@redhat.com>
Cc: Graff Yang <graff.yang@gmail.com>
Cc: Sonic Zhang <sonic.adi@gmail.com>